1. Field of the Invention
The invention is related to flow distributors for enhancing distribution of the reactor hydrocarbon effluent in the channel head of alkylation reactors to improve reactor refrigeration and to minimize tube fouling and corrosion. In the general case, the invention may be applied to improve the distribution of liquid in the channel head of any heat exchanger that has a combined liquid/vapor stream entering the channel head.
2. Description of Related Art
In the alkylation process, isobutane is reacted with light olefins in the presence of a sulfuric or hydrofluoric acid emulsion. Depending on the carbon number and isomer configuration of the olefin molecule, a branched chain isoparaffin results whose octane number ranges from the high 80's to 100. In the sulfuric acid process the reaction usually takes place at 40-50° F., requiring refrigeration.
A schematic of the process known to those skilled in the art is shown in FIG. 1 where liquid phase olefins 10 and isobutane 12 are continuously fed to the reactor 14 at about 40° F. An emulsion of immiscible hydrocarbon and sulfuric acid is formed in the reactor 14 by a mixing impeller 16. The impeller 16 also circulates the emulsion across chilling tubes (not shown) in the reactor 14. The emulsion is forced up from the reactor 14 through line 17 to a settler 18 where the hydrocarbon and acid phases separate. The acid is then returned to the reactor 14 by gravity through line 20. The hydrocarbon phase, which contains the motor alkylate, excess isobutane, propane and normal butane, is released from the settler 18 through line 19 to a flash valve 22 and proceeds into the inlet nozzle 23 of channel head 24 of the reactor 14. From the inlet nozzle 23 of channel head 24, the hydrocarbon vapor and liquid flows through the tube bundle 26 (see FIG. 2) which may comprise, e.g., 300 to 1000+ U-shaped tubes, and exits through outlet nozzle 25. Part of the excess isobutane and light hydrocarbon vaporizes as the reactor effluent flows through the U-tubes, thereby providing refrigeration to the process. The reactor effluent 30 goes to a flash drum 32. The vapor from flash drum 32 is compressed in compressor 34 and routed through chiller 36 and then via line 37a to separator 37. The propane rich stream 37c from separator 37 is fractionated in depropanizer column 38, with the LPG product stream 38a being purged from the unit. The liquid stream 35a from flash drum 32 is fractionated in deisobutanizer column 35. The bottoms alkylate 35c leaves the unit and is typically routed to gasoline blending. The normal butane side draw stream 35b also leaves the unit. The overhead stream 35d, rich in nonreacted isobutane, is combined with the isobutane-rich stream 37b from separator 37 and stream 38b from depropanizer column 38 and recycled to reactor 14.
The industry standard design for an alkylation reactor tube bundle assembly may comprise hundreds of U-shaped tubes. A typical channel head 24 with tube assembly 26 is shown in FIG. 2. A partition 24a in the channel head 24 separates the channel head into an inlet side 23a and an outlet side 25a. The reactor effluent vapor/liquid flow from flash valve 22 enters through an inlet nozzle 23 and proceeds into the inlet chamber 23a of channel head 24. There are several fundamental flaws with this design. The two-phase flow enters the inlet chamber 23a of channel head 24 with reasonably high velocity, since it has already begun to flash, and will impinge on the partition 24a. No provision is made for turning the incoming flow towards the tube sheet 24b on which the tube bundle 26 is mounted. Some of the flow will bend towards the inlet end of the U-shaped tubes (comprising tube bundle 26) that are attached to tube sheet 24b, and some will flow away from the tube sheet into the semi-elliptic head 24. The flow that circulates into the inlet side 23a of head 24 will curl back, pass around the rising inlet jet (from inlet nozzle 23) and enter the U-shaped tubes that are located on the outside, or periphery, of the tube bundle 26. Since the liquid phase is significantly more dense than the vapor phase, the bulk of the liquid will either turn towards the middle of the inlet tubes of tube sheet 24b or will impinge on the partition 24a. The vapor that curls back from the inlet side 23a of head 24 and flows through the outside tubes of tube bundle 26 is deficient in liquid and hence will have significantly lower heat transfer. Industry experience shows that problems with fouling and tube leaks are generally observed in the outside tubes, confirming the flow maldistribution.
Inventions incorporating internal vanes that divide the incoming stream into a plurality of streams have also been reported in the literature. (See U.S. Pat. Nos. 5,531,266, 5,625,112 and 5,811,625, the teachings of which are incorporated herein by reference.)
Division of the incoming stream into a plurality of streams has also been reported by the use of a cluster of small diameter tubes placed inside the channel head of the reactor.
The use of flow distributors as described herein to improve refrigeration and to increase tube bundle life through minimizing corrosion in alkylation processes has not been reported in the open literature.
This same device may be used to improve the distribution of liquid in a two-phase inlet stream on the tube sheet of any similarly configured heat exchanger channel head. When uniform vaporization of the incoming liquid fraction is desired in any of the various shell and tube heat exchangers, such as fixed tube sheet, U-tube, floating head, etc., the instant invention will promote said uniform distribution. The uniform distribution will enhance the vaporization of the liquid and the heat transfer in the exchanger.